This invention relates to x-ray collimators for use in computed tomography systems and the like and specifically to a collimator for precisely controlling an x-ray fan beam.
Computed tomography systems, as are known in the art, typically include an x-ray source collimated to form a fan beam directed through an object to be imaged and received by an x-ray detector array. The x-ray source, the fan beam and detector array are orientated to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane". The x-ray source and detector array may be rotated together on a gantry within the imaging plane, around the imaged object, and hence around the z-axis of the Cartesian coordinate system. Rotation of the gantry changes the angel at which the fan beam intersects the imaged object, termed the "gantry" angle.
The detector array is comprised of detector elements each of which measures the intensity of transmitted radiation along a ray path projected from the x-ray source to that particular detector element. At each gantry angle a projection is acquired comprised of intensity signals from each of the detector elements. The gantry is then rotated to a new gantry angle and the process is repeated to collect an number of projections along a number of gantry angles to form a tomographic projection set. Each acquired tomographic projection set may be stored in numerical form for later computer processing to reconstruct a cross sectional image according to algorithms known in the art. The reconstructed image may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
The x-ray source is ordinarily an x-ray "tube" comprised of an evacuated glass x-ray envelope containing an anode and a cathode. X-rays are produced when electrons from the cathode are accelerated against a focal spot on the anode by means of a high voltage across the anode and cathode. The x-rays produced by the x-ray tube diverge from the focal spot in a generally conical pattern. A fan beam is formed by passing the x-rays through a slot flanked by x-ray opaque material. The process of restricting the x-ray beam to the desired fan beam is termed "collimation" and the slot assembly is termed a "collimator".
A collimator is typically comprised of two opposing metallic blades that may be opened and closed to change the width of the slot and hence to produce a fan beam with varying "thickness", as measured along the z-axis. Alternatively, the blades may be moved in the same direction to displace the centerline of the slot and hence change the fan beam angle with respect to the z-axis. Such a collimator will be termed an "adjustable blade collimator".
It is important that the fan beam have a uniform thickness. Variations in fan beam thickness will cause different detector elements in the detector array to receive different amounts of x-ray radiation despite possible constant attenuation of the imaged object. Generally, such variations in exposure of the detector elements, other than that those caused by the attenuation of the x-ray beam by the imaged object, will produce image artifacts and reduce the dynamic range of the reconstructed image.
When the fan beam is very narrow, uniform thickness of the fan beam is increasingly critical. Small absolute variations in fan beam width create large percentage changes in the exposure between detector elements. Such variations in fan beam width may result from collimator blades that are not parallel.
Motion of the focal spot of the x-ray, primarily the result of thermal expansion of the anode support structure as the x-ray source heats up, will affect the alignment of the fan beam with the imaging plane. The mathematics of image reconstruction assumes that each acquired projection is taken within a single plane. Lack of parallelism of the fan beam with the imaging plane will produces shading and streak image artifacts in the reconstructed image.
Both "ionization" type detectors and "solid state"detectors, as are known in the art, also exhibit changes in their sensitivity to x-rays as a function of the position of the fan beam along their surface. Accordingly, movement of the fan beam as a result of thermal drift of the focal spot may change the strength of the signal from the detector array. Such changes in signal strength during the acquisition of a tomographic projection set produce ring-like image artifacts in the resultant reconstructed image.
Copending application serial number U.S. Pat. No. 4,991,189 entitled: "Collimation Apparatus for X-ray Beam Correction"and assigned to the same assignee as the present invention, teaches the correction of the alignment of the fan beam with the detector array and the imaging plane by movement of the collimator slot along the z-axis direction. In such a system, it is desirable that the center of the collimator slot may be accurately translated along the z-axis to compensate for thermal drift of the focal spot. For the reasons described above, such z-axis translation should occur without changing the fan beam width or affecting the fan beam parallelism.
As previously mentioned, the gantry is rotated about the imaged object and the collimator is fixed relative to the gantry. Accordingly, the collimator experiences a constantly changing force of gravitational acceleration as well as other forces incident to such rotation. It is important, therefore, that a collimator also be able to resist such forces without adverse change in the fan beam's position or parallelism.